Mechanical ventilatory support is widely accepted as an effective means for mechanically assisting or replacing spontaneous breathing. Mechanical ventilation can be non-invasive, involving various types of face masks or nasal devices, or invasive, involving endotracheal tube (ETT) or tracheal intubation. Selection and use of appropriate ventilatory techniques require an understanding of pulmonary mechanics.
Normal spontaneous inhalation generates negative intrapleaural pressure, which creates a pressure gradient between the atmosphere and the alveoli, resulting in air inflow. During mechanical ventilation, the inspiratory pressure gradient is normally the result of or augmented by increased (positive) pressure of the air source. For patients requiring ventilatory support it is necessary to monitor both CRS and RRS to properly assess and treat the patient's pulmonary dysfunction or respiratory failure. Monitoring Pplt is common practice to ensure the lung is not damaged via overdistention or over-pressurization during mechanical ventilation.
RRS is the amount of pressure required to force a given flow of gas though the combined series resistances of the breathing circuit, ETT resistance, and physiologic airways of a mechanically ventilated patient. CRS is a measurement of the distensibility of the lung, meaning the elastic recoil of the lungs and the chest wall for a given volume of gas delivered. Thus, for any given volume, elastic pressure is increased by lung stiffness (as in pulmonary fibrosis) or restricted excursion of the chest wall or diaphragm (i.e., tense ascites, massive obesity). Typically, CRS and RRS are calculated using an end inspiratory pause (EIP) during a constant inspiratory flow rate. CRS is estimated by dividing the delivered tidal volume by inspiratory Pplt, where Pplt is the steady-state pressure measured during an EIP.
RRS is estimated by dividing the difference between peak inflation pressure (PIP) and Pplt by the inspiratory flow rate. Some ventilators have an inspiratory flow rate setting such that the clinician can read the delivered flow rate while others dve an inspiratory time setting where the clinician needs to divide the tidal volume by the inspiratory time to determine the inspiratory flow rate.
Thus, Pplt is essential for calculating CRS and RRS. Moreover, monitoring Pplt is also essential to avoid the over-distension of the alveoli, thus avoiding baro- and/or volutrauma, especially in patients with restrictive lung diseases (ARDS network protocol (July 2008); http://www.ardsnet.org/node/77791). In determining Pplt, current practice requires an EIP be performed. For patients with respiratory failure, this can be accomplished by applying an EIP immediately following a tidal volume during controlled mechanical ventilation (CMV) or intermittent mandatory ventilation (IMV).
Unfortunately, there are many drawbacks to having to perform an EIP. For one, the duration of EIP must be preset by a knowledgeable clinician and applied during mandatory breaths only while monitored. Temporary disruption of inhalation and prevention of exhalation by applying an EIP can be uncomfortable for some patients, causing the patient to involuntarily or voluntarily make active inspiratory or expiratory muscle contractions at the time of EIP, which can affect the accuracy of measured Pplt. If an imprecise measurement for Pplt is obtained, resultant estimations for CRS and RRS would also be inaccurate. Because, as noted above, patient respiratory therapy and treatment are based on CRs and RRS values, erroneous calculations for CRS and RRS due to imprecise Pplt measurements could subsequently affect the efficacy of treatment delivered to the patient and the patient's recovery, perhaps even to the detriment of the patient's health.
Further, because performing an EIP can be uncomfortable to the patient, it cannot be applied continuously. Without continuous, accurate Pplt information, the clinician is unable to fully monitor patient safety and the efficacy of treatment.
Temporary disruption of inhalation by applying an EIP may also predispose to patient-ventilator dysynchrony. This may lead to increased work of breathing, and the possibility of compromising arterial blood-gas exchange.
Finally, an EIP may not be applied (or may be inaccurate) during pressure support ventilation (PSV), continuous airway procedure (CPAP), or other ventilatory modes which do not employ a constant inspiratory flow rate during the inhalational phase. Because of the inability to apply an EIP in these situations, clinicians are precluded from accurately assessing a patient's Pplt, CRS and RRS when ventilated with these modalities. Without correct assessment of patient Pplt, CRS and RRS, efficacy of therapy and/or appropriate diagnosis of pulmonary disease or condition cannot be determined.
Thus, repeated accurate measurements of Pplt, and therefore CRS and RRS, are difficult to obtain using EIP. If Pplt could be determined without the need of applying an EIP, then more accurate estimations of CRS and RRS can be performed, even in real-time, precluding the need to interrupt the inhalation phase. Such an approach would be simpler and preferred, for both the clinician and the patient, and therefore needed in clinical practice.
In addition, knowledge of Pplt during PSV would provide continuous monitoring of CRS and RRS, and preclude the need to change ventilator modes.
Accordingly, there is a need in the art for a system and method to noninvasively, in real time accurately calculate Pplt, CRS, and RRS without the need to modify ventilator inspiratory flow waveform pattern which may cause adverse effects such as patient-ventilator dysynchrony. A continuous, real time, and accurate understanding of the effects of mechanical ventilation and other therapeutic interventions (for example bronchodilators and airways suctioning) on pulmonary mechanics (i.e., CRS and RRS) is needed to promote patient-ventilator synchrony and arterial blood-gas exchange. The present invention is designed to address this need.